In Situ Generation and [3 + 2] Annulation Reactions of Propiolaldehyde?A Metal-Free, Cascade Route to Pyrazole and Bipyrazole Carboxaldehydes in One Pot

Sreelekha, M.K.Shamnad, A.Bhaskaran, R.P.Babu, B.P.2026-02-032025Journal of Organic Chemistry, 2025, , , pp. -223263https://doi.org/10.1021/acs.joc.5c00246https://idr.nitk.ac.in/handle/123456789/20704Propiolaldehyde, the most reactive yet less explored electrophilic acetylene, was generated in situ via a base-free Kornblum oxidation from propargyl tosylate and successfully intercepted with hydrazones affording pyrazole-4-carboxaldehyde in one-pot by a [3 + 2] annulation reaction. Further, the pyrazole-4-carboxaldehyde endured a unique cascade reaction with phenylhydrazine and propargyl tosylate, yielding synthetically challenging bipyrazole carboxaldehydes. The method is free of any metal catalyst, base, or additives and offers a convenient protocol to handle the reactive and volatile propiolaldehyde under ambient conditions. © 2025 American Chemical Society.Annulation reactionsBase-freeBipyrazolesCascade reactionsHydrazonesMetal freeOne potPropargylPyrazolesSitu generationIn Situ Generation and [3 + 2] Annulation Reactions of Propiolaldehyde?A Metal-Free, Cascade Route to Pyrazole and Bipyrazole Carboxaldehydes in One Pot